The interests of this laboratory continue to focus on the relationship of bile pigment metabolism to the production and destruction of heme, hemoglobin, and red blood cells. Studies are concerned with the pathophysiology of the early-labeled fraction of bile pigment which appears soon after the administration of a labeled precursor such as glycine-2-C14 and is derived from sources other than the breakdown of hemoglobin in senescent red blood cells. Emphasis is given to the mechanisms and implications for disease states of bilirubin production both from nonhemoglobin sources in the liver and from processes related to red blood cell production and hemoglobin synthesis. Investigations of the following specific problems are proposed: the nature and significance of early-labeled heme in both hepatic and erythroid cells; alterations in hepatic and erythroid heme metabolism under pathologic conditions; possible relationships between the regulation of synthesis of non-hemoglobin hemes in the liver and hemoglobin heme in the erythroid cells; mechanisms of bile pigment excretion by the liver; alternate pathways of bilirubin excretion; and the effects of phototherapy on bilirubin production and excretion, with particular attention to red cell hemolysis.